| William Augustus, Duke of Brunswick-Harburg | |
| Noble Family: | House of Guelph |
| Father: | Otto II, Duke of Brunswick-Harburg |
| Mother: | Hedwig of East Frisia |
| Birth Place: | Harburg |
| Death Place: | Harburg |
Duke William Augustus of Brunswick-Lüneburg-Harburg (15 March 1564 in Harburg - 30 March 1642 in Harburg) was Duke of Brunswick-Lüneburg-Harburg.
William Augustus was a son of Duke Otto II of Brunswick-Harburg (1528-1603) from his second marriage with Hedwig (1535-1616), the daughter of the Count Enno II of East Frisia.
William Augustus was regarded as extremely learned and, like his father, as a follower of the Lutheran doctrine. In 1575, he became rector of the University of Rostock. He later continued his studies at the University of Leipzig. In 1582, he undertook a Grand Tour to France and England and then enrolled with his brothers at the University of Helmstedt. In 1594, he again went on a journey that took him through Germany, Poland, Switzerland, Italy, Holland, Denmark and Livonia. William Augustus kept a journal about his travels.
After the death of his father, he took up the government of Harburg jointly with his brothers Christopher and Otto III. After they died, he ruled alone.
In 1618, William Augustus began the construction of the palace at Moisburg. After the death of the Duke Frederick Ulrich of Brunswick-Wolfenbüttel, he received the County of Hoya from his inheritance. August Wilhelm died unmarried and childless. His inheritance was divided by Dukes of Frederick IV of Brunswick-Celle and Augustus II of Brunswick-Wolfenbüttel.